Our overall interest is in advancing the understanding of calcium homeostasis in vertebrates. In the current proposal, one of the principal control systems involved in calcium homeostasis, the renal-vitamin D endocrine system, will receive the major consideration. A comparative approach will be used involving both mammalian and submammalian vertebrates, represented mainly by the rat and the Japanese quail respectively. Nevertheless, poilkilotherms such as amphibia and fish will also receive attention. The major thrust will be concerned with the mechanism, both physiological and pharmacological, by which the renal-vitamin D endocrine system is regulated in avian and mammalian species. Initially the studies will focus on developing our earlier observation that estrogens influence the vitamin D endocrine system in Japanese quail. Nevertheless, other endocrine and nutritional factors (parathyroid hormone; prolactin; dietary vitamin D, calcium and phosphorus) will also receive attention. As our understanding of the mechanisms underlying these regulatory influences in homothermic vertebrates increases, more limited studies will be initiated in amphibian and piscine species in order to determine how widespread such mechanisms are distributed throughout the vertebrate phyla.